Inkjet profiling ABC_1
Inkjet profiling ABC_1
- Subject: Inkjet profiling ABC_1
- From: Henrik Holmegaard <email@hidden>
- Date: Sun, 14 Apr 2002 09:38:39 +0100
I seem to get off-line requests for this post fairly often, so I'll
repost it. Sorry for the iteration, but it's easier this way.
---
Just as the desktop learnt to compose type and composite images, it
will soon be teaching black generation to press operators. The reason
is simple, there are more people and more inkjets involved than there
are presses and press operators. And more people can teach each other
faster than fewer people can because the pool of experience is wider.
But I think it's easier to catch on if you step backwards from the UI
logic and look at the reason say ProfileMaker or Printopen show much
the same total CMYK and black generation presets. If you know enough
about what sort of printing mechanism an inkjet is, you can audit the
UI presets and make independent decisions.
So why do profiling applications have presets for total area coverage
at max 260% and a very light black channel which doesn't touch the
highlights and midtones? An offset press will run much higher total
area coverage, and as inkjets are wet printing systems, surely there
are savings to be made on CMY(cm) inks by using K instead?
a. To fire a drop of ink, an inkjet reckons with drop placement
error. Say if the resolution is 1/600 x 1/600 of an inch, then the
drop isn't as one might expect the diameter of the cell, but a little
larger than the diagonal. This way at the same percentage fed to an
offset system and an inkjet system, the inkjet will lay down more ink.
b. An offset printing system has better placement in the sense that
spatially the individual printing plate is precision imaged and
dimensionally stable, but on the other hand the color-to-color
registration may be better on an inkjet where the printhead array is
tighter co-ordinated in both space and time.
c. Going from things mechanical to things chemical, offset systems
lay down thin layers of fat-based pigment inks on top of each other.
Inkjet inks use non-fatty vehicles and the ink must penetrate the top
coating of the media. Once that is full, the ink bleeds to
neighbouring areas.
d. Density control comes before color control. Ink limiting is about
going high to get the pure colors that outline the gamut volume, and
not loosing detail through spatial bleeding problems. Taking down the
ink limit shrinks gamut and boosts detail. Taking up the ink limit
boosts gamut and shrinks detail. Before the CMYK testchart is printed
as deviceCMYK (no conversions anywhere), get the ink limit right, and
make it stay right.
e. Some inkjet systems are four color CMYK, others six color CMYKcm
with finer highlight screening. Colorimetrically the CMYKcm printers
are CMYK printers (depending on the hue of the c and m inks). In
terms of ink densities both printer types present a CMYK interface to
the world as the media chemistry is the same. So on a six color
inkjet the total area coverage is divided over six colors rather than
four causing each color to have a lower limit, except that in the
light colors more ink is put down for the same L density than in the
dark colors, so here in the light colors ink densities rise. This is
handled internally in CMYKcm inkjets.
f. On an offset printing system K is used to replace CMY in order to
economize, to reduce the impact of color variation, and to control
spatial problems. On an inkjet this strategy doesn't work, or rather
a different strategy applies. As a rule an inkjet separation should
have as light black in highlights and midtones as possible where an
offset separation might have GCR throughout the color space - for
CMYKcm inkjet systems a lower black than for CMYK inkjet systems.
ProfileMaker pushes the black start for its default GCR inkjet
separation way down, for instance. And if you look at bundled inkjet
profiles, the same rule of very light black is evident.
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